Well, I think the entire reason the US has been so quick to insert itself in situations like this is because there's a very real feeling that the US can't really lose these wars, because the consequences of failure are very different from, say, WW2.

If the stakes were higher for America as a nation we'd probably be significantly less likely to enter into these sorts of conflicts to begin with. And if the stakes were higher for America in the conflict for some other reason - say it was Mexico instead of Vietnam - there'd probably be significantly more political and public support for continued action and additional resources being thrown at it. But it's hard to do that when it's for the more nebulous idea of keeping the foreign power most aligned with US interests in the region in power.

Which isn't to say I disagree with the general idea - Vietnam was a loss by any reasonable sense of the word - but that I think there is a certain amount of truth to the idea that people in power really did look at Vietnam as a war that couldn't truly be lost.

World war 1 is not the same as world war 2.